Steel is made by melting and refining iron and steel scrap in an electric arc furnace (EAF). Today, the EAF is considered by those skilled in the art of steel production to be the single most critical apparatus in a steel mill or foundry. Consequently, it is of vital importance that each EAF remain operational for as long as possible.
Structural damage caused during the charging process affects the operation of an EAF. Since scrap has a lower effective density than molten steel, the EAF must have sufficient volume to accommodate the scrap and still produce the desired amount of steel. As the steel melts it forms a hot metal bath in the hearth or smelting area in the lower portion of the furnace. As the volume of steel in the furnace is reduced, however, the free volume in the EAF increases. The portion of the furnace above the hearth or smelting area must be protected against the high internal temperatures of the furnace. The vessel wall, cover or roof, and duct work are particularly at risk from massive thermal, chemical, and mechanical stresses caused by charging and melting the steel. Such stresses greatly limit the operational life of the furnace.
Historically, the EAF was generally designed and fabricated as a welded steel structure which was protected against the high temperatures of the furnace by a refractory lining. In the late 1970's and early 1980's, the steel industry began to combat such stresses by replacing expensive refractory brick with water-cooled roof panels and water-cooled sidewall panels located in portions of the furnace vessel above the smelting area. Water-cooled panels have also been used to line furnace duct work. Existing water-cooled panels are made with various grades and types of plates and pipes.
Using water-cooled panels has reduced refractory costs and has also enabled steelmakers to operate each furnace for a greater number of heats. Furthermore, water-cooled equipment has enabled the furnaces to operate at increased levels of power. Consequently, production has increased and furnace availability has become increasingly important.
Although water-cooled panels last longer than brick refractory, the panels have problems with wear and are subject to damage. Critical breakdown of one or more of the panels commonly occurs within a few months of furnace operation. When such a breakdown occurs, the EAF must be taken out of production for unscheduled maintenance to repair the damaged water-cooled panels. Since molten steel is not being produced by the steel mill during downtime, opportunity losses of as much as five thousand dollars per minute for the production of certain types of steel can occur. In addition to decreased production, unscheduled interruptions significantly increase operating and maintenance expenses.
To increase the life of water-cooled components, an effort is made to promote slag adherence to the surface of the water-cooled equipment. Adhered slag "freezes", that is solidifies, to the water-cooled equipment thereby forming a thermal and chemical barrier between the cooling equipment and interior of the furnace.
In prior art furnaces, slag is encouraged to stick to the cooling equipment by welding studs, fins or cup like members onto the surface of the equipment, or by using slag bars or other similar items. For example, U.S. Pat. No. 4,221,922 discloses a fin welded to a water-cooled panel. However, these typical methods cause stress risers, that is, the beginning of cracks at the molecular level within the material of the water-cooled pipes. The stress risers are caused by localized heating differentials or stress differentials during the manufacture of the pipes. As an electric arc furnace cycles, the components expand and contract, further breaking down the grain structure in the material of the pipes and broadening the stress risers, until a pipe in the cooling apparatus fails prematurely. Water leaking from a damaged pipe into the furnace can potentially lead to catastrophic reoxydation of hot metal in the furnace. Hence, a damaged cooling element must be timely replaced.
A need, therefore, exists for an improved water-cooled furnace panel apparatus which remains operable longer than existing comparable panels and continues to operate, despite some structural damage, until scheduled maintenance occurs.